A direct detection receiving manner is used in conventional optical communications. However, as a communication capacity increases, a problem such as chromatic dispersion, frequency offset mode dispersion, a nonlinear effect, or phase noise occurs in a fiber link, and the conventional direct detection receiving manner is gradually replaced by an optical coherent detection technology.
Because an optical coherent receiving manner may support multiple modulation formats, by taking full advantage of information about light, such as an amplitude, a phase, or polarization, spectrum utilization may be improved, and a fiber transmission capacity may be increased. FIG. 1 is a schematic structural diagram of a coherent optical communications system. In the coherent optical communications system, a signal laser at a transmit end generates signal light, and the signal light enters a receive end after being modulated by a modulator. A local oscillator laser at the receive end generates local oscillator light. Frequency mixing is performed on the signal light and the local oscillator light by using an Integrated Coherent Receiver (ICR). A frequency offset, between the signal light and the local oscillator light is compensated in an electric domain by using a Digital Signal Processor (DSP). Currently, a frequency offset compensation capability of a DSP in a coherent system is +/−5 GHz. Therefore, requirements of the coherent system for frequency stability of both the signal light and the local oscillator light are +/−2.5 GHz, and in addition, it is required that frequency offsets between wavelengths of the signal light and the local oscillator light and an International Telecommunication Union (ITU) standard wavelength are separately controlled within +/−2.5 GHz. The signal laser at the transmit end in the coherent system and the local oscillator laser at the receive end each include a wavelength locker. Feedback control from the wavelength locker and a wavelength locker algorithm unit ensures that the frequency offsets between the wavelengths of the signal light and the local oscillator light and the ITU standard wavelength are separately +/−2.5 GHz, that is, a frequency offset between the signal light and the local oscillator light is +/−5 GHz. This frequency offset value falls within a range of the compensation capability of the DSP.
However, due to its relatively high costs, the coherent optical communications mainly targets long-distance optical transmission. Therefore, costs of the coherent system need to be reduced, and the coherent system needs to be applied to a metropolitan area or an access domain. Respectively omitting the wavelength lockers in the lasers at the transmit end and the receive end may reduce the costs of the coherent system. Currently, there is a laser without a wavelength locker: a distributed feedback (DFB) laser. If the laser without a wavelength locker is used, wavelength control precision is generally +/−0.1 nm (frequency stability is +/−12.5 GHz), and a frequency offset between the signal light and the local oscillator light reaches +/−25 GHz. However, currently, the frequency offset compensation capability of the DSP in the coherent system is +/−5 GHz, and consequently, using a laser without a wavelength locker in the coherent system cannot implement frequency offset compensation.